Bit mapped displays treat the display screen as a discrete rectangular mesh or matrix of dots. These dots are referred to as pels or pixels (picture elements), and each pel is represented by one or more bits in the display's memory array. A very useful function for bit mapped displays is the ability to move a rectangular block of pels from one place in the bit map to another place and logically combine the source rectangle with the destination rectangle. These logical functions include AND, OR, EXCLUSIVE OR, REPLACE, etc. This function can be accomplished without any assistance from the hardware, but this is costly in code space and execution time.
Bit addressability is the ability to start reading or writing a series of pel data at any particular pel or pixel location in memory. This ability was limited to a horizontal series of pels because of the traditional layout and control of the bit map's memory, but it did help relieve some of the burden from software.
An example of an image processing memory is disclosed in U.S. Pat. No. 4,090,174 entitled "Method and Apparatus for accessing Horizontal Sequences, Vertical Sequences and Rectangular Subarrays from an Array Stored in a Modified Word Organized Random Access Memory System" assigned to the present assignee.